Takashi Nagata

A highly productive basin-type-multiple-effect coupled solar still

Abstract A newly designed multiple-effect solar still with a triangle cross-section consisting of a horizontal basin liner, a tilted double glass cover and the vertical parallel partitions in contact with saline-soaked wicks is presented and theoretically analyzed. Solar radiation is absorbed in the basin and in the first partition, and the partition section recycles the energy from the basin as well as solar energy, which is directly absorbed by the first partition. A single distillation cell with a 5-mm diffusion gap between vertical partitions has been experimentally explored. No contamination of distillate with saline water was experimentally detected in the single distillation cell, and the experimental observation of the wick and the measurements of the temperature drop through the cell with the 5-mm gap showed that dry patches hardly appear on the wick. The proposed still with 5-mm diffusion gaps is theoretically predicted to produce distillate of 15.4kg−2 m−2d−1 on a sunny day of Gti=22.4 MJ/m−2d−1 solar radiation, and its efficiency is about 3.5 times larger than the average experimental value for the conventional basin type stills by Cooper [20] and 1.2-1.6 times larger than the experimental maximum values of the conventional multiple-effect stills [3,9].

Enhancement of mass transfer into a falling laminar liquid film by two-dimensional surface waves—Some experimental observations and modeling

Two-dimensional surface waves were produced on a falling laminar water film by input of controlled disturbances, and the increase in the film absorption of the surrounding gas was determined at temperatures, 17.1–18.4°C. The augmentation decreases constantly with wave frequency for low Reynolds numbers Re 20. Taking account of these experimental results and the numerical observations by Nagasaki and Hijikata (1990, Proceedings of the 3rd Japan Mechanical Engineering Society Symposium on Numerical Dynamics, No. 900-69, pp. 47–48) and others, we constructed a double boundary layer model, in which the mass transfer coefficient kL is expressed as kL=ξ3.46Dfwπ, where D and fw are the diffusivity in the film and the frequency of the waves, respectively. The equation, which is derived from the penetration theory, introduces the parameter ξ for waves sweeping highly concentrated thin layers on the surface. It was found that at constant physical properties of liquid the parameter is a function only of phase velocity, and thus increases with phase velocity.

Characteristics of two-dimensional waves on a falling liquid film

Regularly spaced two-dimensional waves are excited with input of controlled disturbances to a water film falling down a vertical surface, and phase velocities uw, wave peak heights hp and wave separations λ are measured for a wide range of flow rates (Γ = 20 ∼ 86 × 10−6 m2/s) and temperatures (5 ∼ 23°C) (corresponding to Reynolds numbers Re = 14 ∼ 90). The variables hp and uw are well described by the relations Nuw = 1.13KF0.02Nλ0.31Re0.37 and Nhp = 0.49KF0.044Nλ0.34Re0.46, and also by Nhp = 0.425KF0.019Nuw54 and Nuw = 1.13KF0.026NV12 where Nuw,Nhp.Nλ.KF.NV and Re are respectively dimension-less phase velocity, wave peak height, wave separation, physical properties group, volume of liquid carried on a wave and Reynolds number. These correlations show that the two variables λ and Γ can be replaced by one variable, V or uw, and they therefore indicate that the waves are isolated from each other. Comparisons between these correlations and data obtained from the extensive literature on the subject are discussed in relation to the surface elasticity. Shadowgrams of waves are also presented.

Experimental study of basin-type, multiple-effect, diffusion-coupled solar still

Abstract Basin-type, multiple-effect, diffusion-coupled solar stills consisting of a tilted double glass cover, a horizontal basin liner and a number of closely spaced vertical partitions in contact with saline-soaked wicks were constructed and then examined in an outdoor experiment in fall and winter at Okinawa, Japan. This new type of still is greatly improved by narrowing diffusion gaps between the partitions, increasing the number of partitions, sandwiching small spacers between partitions and insulating the frame of the glass cover. The daily performances of the improved still are in good agreement with predictions calculated from the theoretical model developed by Tanaka et al. [1]. The still with 5-mm diffusion gaps between 11 partitions produces 14.8–18.7 kg/d distillate per unit effective area of the glass cover at 20.9–22.4 MJ m−2 d−1 solar radiations incident on the glass cover and 19–30°C ambient air temperatures and is highly productive in comparison with the 7-effect diffusion-type still with 8-mm gaps developed by Okamura et al. [2] and the 3-effect diffusion-type still with 19-mm gaps of Cooper and Appleyard [3].

Parametric investigation of a basin-type-multiple-effect coupled solar still

Abstract Parametric analysis was performed for the basin-type multiple-effect coupled solar still with a triangle cross-section consisting of a horizontal basin liner and a number of vertical parallel partitions in contact with saline-soaked wicks with narrow gaps between the partitions, under dry weather conditions at 26.1 °N latitude. The productivity of the still increased in winter with an increase in the angle between the glass cover and the basin and decreased in summer. In the spring and autumn seasons productivity had a gentle peak between a 40° and 45° angle. Increasing supply rates of saline water to the wicks and increasing initial mass of the basin water decreased productivity. With a decrease in diffusion gaps between partitions, productivity exponentially increased, and it also showed exponential increases with an increase in the number of distillation cells between the partitions. The productivity of the still of 13 partitions with 5-mm gaps and a 40° angle of the glass cover was four times more than the basin-type stills, and the still was more productive than the conventional multiple-effect stills by about 40% or more.

A new maritime lifesaving multiple-effect solar still design

Abstract This paper presents a new maritime lifesaving multiple-effect solar still design where several extended wicks feed seawater to their evaporating areas by capillary force, and the resulting water vapor diffuses and condenses on facing wicks with the condensate flowing through the wicks into storage bags. A theoretical analysis showed that a temperature drop through diffusion layers between evaporating and condensing wicks slowly increases in the main evaporating areas and rapidly near salt depositions whereas the evaporation rate decreases slowly in the main area and rapidly near salt depositions. These changes are caused by increases in salt concentrations and resulting boiling point elevation. With steady-state transfer analysis, the proposed still is predicted to produce about 15 kg m −2d−1 fresh water on a sunny day of 22 MJm−2d−1 solar radiation, showing a potential to be a maritime lifesaving desalinator.

Characteristics of falling film flow on completely wetted horizontal tubes and the associated gas absorption

Abstract Falling water films on completely wetted horizontal tubes of 16 mm diameter in a vertical row and the associated oxygen gas absorption were experimentally investigated in the Reynolds number range of 10 30. When Ls=5 mm or larger, dripping from each tube occurs and causes the larger amplitude waves which rapidly spread on the film on the lower tube. The Sherwood number, Sh, greatly increases with an increase in Ls from 2 to 5 mm and then levels off at Ls=10 mm or larger. When Ls=2 mm, Sh is in proportion to Re1.15 for Re 30. The Sh varies as Re0.86 for the whole Re range at Ls=5–15 mm. Comparison of the present results with empirical equations for vertical tube absorber indicates that volume of horizontal tube absorber can be smaller than that of vertical tube absorber by a factor of 1/1.18–1/2.2.

Indirect Cooling of IC Chips Using a Two Phase Closed Thermosyphon Loop

This paper reports on indirect cooling of high-power IC chips of notebook computers using a two phase closed thermosyphon loop with Fluorinert (FC-72) as the working fluid. The experimental set-up consists of an evaporator and a condenser connected by flexible tubing. The evaporator corresponds to a high-power IC chip, and the condenser represents a cooling plate located behind the display of notebook computer. The evaporator and the condenser have the outer dimensions of 50mm x 50mm x 20mm and 150mm x 200mm x 20mm, respectively. The effects of the heat input Q and the charged volume of Fluorinert liquid F on the heat transfer characteristics of the cooling system were studied experimentally. Further, the experiment for the evaporator with plate fin to enhance the boiling in the evaporator was carried out. It has been confirmed that the heater surface temperature for the evaporator with plate fin reduces about 10% in comparison with those for the evaporator without fin. It is found that enhancing the boiling in the evaporator is very effective to reduce the surface temperature of heater. In the case of the evaporator with the plate fin, the temperature difference between the heater surface and ambient is kept around 60K for the highest heat input Q=30W in the present experiments.

AN EXPERIMENTAL STUDY OF MASS TRANSFER FROM FLUTTERING FLAGS

Mass transfer from a whole flag with free trailing edge fluttering vigorously with large amplitude in vertical wind and from parts of flags fluttering with arc-shapes between fixed leading edges and nodes was experimentally investigated in the ranges of the Reynolds number Re = 3 × 10 4 -4. 5 × 10 4 and 1 × 10 4 -5 X 10 4 , respectively. The arc-shape fluttering causes an increase in the Sherwood number only by 15% whereas it has fluttering frequencies much larger than the vigorous fluttering with the free trailing edge, which causes a rapid increase in the Sherwood number with wind velocity up to 3.2 times what is measured for the flags being still in the wind. The abrupt 14-fold increase in the drag of flags due to fluttering and the smoke flow visualization presented by Taneda and the 2-6-fold increase in heat transfer from a flat surface due to flow separation and reattachment measured by Ota and Kon strongly indicate that flow separation and reattachment occur on the vigorously fluttering flag to cause the 3.2-fold increase in the Sherwood number

The Refracted Image Moire Topography for Measuring Liquid Film Profiles

The refracted image moire topography has been modified to measure a film profile having large curvatures. An optical system for the modified topography has been built to produce moire fringe patterns, and a method reconstructing the film profile from the fringe has been developed. A planoconvex lens and a wavy liquid film falling down a vertical plate have been examined with the optical system and the reconstructing method. The results have indicated that the modified topography has a good accuracy and its maximum measurable curvature is 60 m-1 in the case of the falling liquid film.